In the art of thermal recording, it is necessary to provide a "head" composed generally of a resistive element which heats up to a predetermined temperature upon the application of electrical energy. The temperature is such as to cause a mark or dot to appear on a thermally sensitive material usually by melting a portion of the material. Special paper is usually employed which may be one of several types. For example, a paper manufactured by the Jujo Paper Company, Ltd., and identified by the number TP50KH-H5 employs a thermoplastic that has dye components that are colorless until heated but, upon the application of heat, turn to a contrasting color such as black. Alternatively, a paper having a thermoplastic surface may be melted and then moved to a position where toner is applied as is disclosed in D. F. Anderson et al. U.S. Pat. No. 3,362,380 which issued Jan. 9, 1968. Another type of paper referred to as a "dye transfer" paper, is manufactured by the General Company, Ltd. and is identified by the number CTTC-F-6M. This paper involves the use of a waxy, colored material that melts upon the application of heat and the melted areas attach themselves to leave marks on a receiving paper placed adjacent to the dye transfer paper. After the deposits have been made onto the receiving paper, it is separated from the dye transfer paper. This is less messy than the application of toners and has the advantage over interior dye component paper of allowing colored reproductions since, after the receiving paper has been removed from a dye transfer paper of a first color, for example yellow, it may be placed against a second dye transfer sheet of a different color, for example magenta, and then yet against a third dye transfer sheet of, for example cyan and the process repeated in each step so as to produce the full range of hues desired.
One type of thermal printing apparatus employs a plurality of individual thermal heaters placed side by side across the width of the paper. Examples of this may be found in a patent to Yuji Oba U.S. Pat. No. 4,140,907 which issued Feb. 20, 1979 and in the Hans-Dieter Hinz U.S. Pat. No. 4,092,650 which issued May 30, 1978. In such apparatus, a plurality of dots or marks are printed across the width of the paper. The paper is then advanced and further dots or marks are printed. This process is repeated until a desired image is produced. In order to get various grey scale values, it is desirable that the dots cover various areas on the paper. Very small dots would leave the paper almost white whereas very large dots could be made to overlap and produce a totally black surface on the paper. Dots of various sizes in between these extremes would produce various grey scale values. One problem that has arisen in the apparatus of the prior art is that of the extreme difficulty in controlling the size of the dots not only accurately but evenly. Dots which are of the wrong size or which are shaped unevenly or oddly will produce grey scale values that are not desired. Attempts have been made to control the size of the dot accurately by shaping the conductors that lead to the resistive thermal printing head as is shown in the Steven Kos U.S. Pat. No. 4,300,142 which issued Nov. 10, 1981. The difficulty encountered with the Kos structure lies in the fact that the conductors are of a material which conducts heat away from the area being melted much faster than the resistive portion of the head. Thus as the melt begins to grow, the heat is conducted away from the paper by the conductors in one direction while it is not so conducted in the other with the result that the dots or marks that are left by the thermal head are oblong or otherwise oddly shaped and produce grey scales of undesired values.